1. Field of the Invention
The present invention relates to a calibration method for protrusion detecting head units in a protrusion inspection device used for magnetic disks which can determine a highly reliable measurement data even if there is dispersion in detection characteristics of the protrusion detecting head units. The present invention also relates to a protrusion inspection method and a protrusion inspection device using the same.
2. Conventional Art
Magnetic disks (hereinafter simply called disk or disks) which constitute one of information recording media are manufactured while using aluminium or glass disks as the base and applying a magnetic film on the respective surfaces thereof. The magnetic film surface is required to be formed into a smooth and flat surface with no unevennesses such as protrusions. For this purpose, the magnetic film surface is polished. However, even after polishing protrusions may remain. When there are more than a predetermined number of protrusions having a height more than a predetermined one, these protrusions collide with a magnetic head and damage the same, and further are likely to cause errors in data to be accessed. In order to prevent these dangers, protrusion inspection for magnetic disks is performed with a protrusion inspection device. If the result of the inspection revealed that more than the predetermined number of protrusions having a height more than the predetermined one still remain, the magnetic disk was polished again.
For the protrusion detecting head unit (hereinafter simply called as head unit) in a protrusion inspection device a piezo-electric element using quartz was used for the first time. However the detection sensitivity and the saturation characteristic of the piezo-electric element are unsatisfactory, and further miniaturization thereof was difficult. Therefore an ultrasonic piezo-electric sensor (hereinafter simply called as ultrasonic sensor) having excellent detection sensitivity and saturation charactristic has drawn increased attention. The inventor of the present application produced a head unit using such an ultrasonic sensor for which a U.S. patent application was filed having U.S. Ser. No. 07/977,634 which is abandoned and now Ser. No. 08/350,034, now U.S. Pat. No. 5,488,857, and entitled "PROTRUSION SENSOR FOR SENSING PROTRUSION ON A DISC".
FIG. 4 shows a schematic view of such head unit in connection with the above U.S. patent application. Numeral 3 designates the head unit which is constituted by a slider 31 functioning as a thin film head, a suspension spring 32 fixedly securing the slider at the top end thereof, a supporting arm 33 supporting the suspension spring while clamping the same at the bottom end thereof and an ultrasonic sensor 34 fixedly secured at a proper portion on the supporting arm 33. The ultrasonic sensor 34 is made of polycrystalline ceramics of barium titanate or lead zirconate titanate and has a specific vibration frequency which is determined depending on the thickness and size thereof.
During a protrusion inspection, when the head of the head unit 3 is loaded with respect to a track TR of the rotating disk 1, the slider 31 is lifted through air flow by a predetermined lifting amount .delta.H. When the slider 31 collides with a protrusion existing on the track TR, the slider is caused to vibrate. The vibration is transmitted to the supporting arm 33 via the suspension spring 32. As a result, the ultrasonic sensor 34 is vibrated at its specific vibration frequency, generates a voltage signal (a collision detection signal) and outputs the same.
FIG. 5 shows a constitutional diagram of the protrusion inspection device. A disk 1 for the inspection object is mounted on a spindle 21 of a rotary mechanism 2 and is driven to rotate by a motor (M1) 22. Head units 3A and 3B are respectively provided with respect to both faces, i. e. the front face (upper side) and the back face (lower side) of the disk 1 (in the embodiment of the above mentioned prior application the head unit 3B is not provided of which difference is explained later) and the respective supporting arms 33, 33 for these head units are clamped at a carriage mechanism 41 of a carriage portion 4. When a motor (M2) 42 is rotated, the slider 31 moves toward the radial direction of the disk 1 and scans the disk 1 while either sequentially stopping at every track TR or continuously moving in a spiral manner. Usually, a carriage control program and a circumferential speed control program are stored in a memory (MEM) 8a in order to keep the lifting amounts .delta.H of the respective sliders 31 constant with respect to the entire tracks TR and an MPU 8 executes both programs to operate a carriage control circuit 6 and a circumferential speed control circuit 7. As a result, the motor M2 is rotated by the former and the motor M1 by the latter and the circumferential speed of the respective tracks TR on the disk 1 is kept constant with respect to slider 31, which performs scanning while either sequentially stopping or moving in spiral manner, to perform the protrusion inspection.
Collision detection signals outputted from the respective ultrasonic sensors 34, 34 when the respective head units 3A and 3B collide with protrusions are respectively inputted to a protrusion detection circuit 5 wherein the level of the input is compared with a predetermined threshold level to perform the protrusion detection. The resultant protrusion detection signal P (see FIG. 6(e)) is processed in the MPU 8 and is stored in the memory (MEM) 8a for the management as data relating to such as collision position and collision number. Then after the measurement, this data is outputted from a printer (PRT) 9 as measured data of protrusions.
FIG. 6(a) is a constitution diagram of the protrusion detection circuit 5. The collision detection signal Vf (see FIG. 6(b)) outputted from the ultrasonic sensor 34 or the piezo-electric element is amplified by an amplifier 51 and unnecessary components therein are removed by a band-pass filter (BPF) 52 to produce a waveform signal Vfc (see FIG. 6(c)). The waveform signal Vfc is subjected to an envelope detection to produce another waveform signal VfL (see FIG. 6(d)). Subsequently the waveform signal VfL is compared at a comparator 55 with a proper slice voltage Sv to convert it into a digital signal to determine the protrusion detection signal P as shown in FIG. 6(e). The protrusion detection signal P is stored at a predetermined position in a buffer memory (BMEM) 57 depending upon its generation timing, and the signal is processed in the MPU 8 at a good time after either reading out the stored data or being transmitted.
Now, the slider 31, the suspension spring 32 and the supporting arm 33, which constitute the head unit 3, have respective specific vibration frequencies depending on criteria such as their weights and elastic characteristics, however these values disperse and their vibrations suffer from irregularity. The ultrasonic sensor 34 itself also shows a deviation with respect to the original specific vibration frequency due to its thickness dispersion. For these reasons, the lifting characteristics of head units with respect to a circumferential speed at a certain track and the output voltages in collision detection signals Vf generated by a collision vary from head unit to head unit, and dispersion of their output voltage characteristics results.
On one hand, in the course of the protrusion detection operation, breaking down and characteristic deterioration of such as the slider 21 and the ultrasonic sensor 34 are often caused due to the collision of the head unit 3 due to protrusions. When such elements deteriorate or break down, the entire head unit 3 is replaced. However, since the replaced head unit contains dispersion in its output voltage characteristic, the replaced head unit will show a detection irregularity. Thus the replaced head unit may or may not detect a protrusion having an identical size of which the head unit before the replacement could detect, thereby reliability of the protrusion inspection operation is lowered.
In particular, in the magnetic disk memory devices which are tending toward development of a high density memory, the result of the protrusion inspection closely relates to reliability of the devices. Therefore the ability to detect a protrusion having a more lower height with a high reliability is required.
Unlike the embodiment disclosed in the U.S. Ser. No. 07/977,634 which is abandoned and now Ser. No. 08/350,034 now U.S. Pat. No. 5,488,857, an example has been explained in which another head unit (the head unit 3B) is provided also at the back face side. In such case where the head units are provided at both front and back face sides it is indispensable to equalize the characteristics of these heads for increasing the inspection reliability in the simultaneous inspection of the front and back face sides. If the characteristics of the head units for the front and back face sides are not equalized, the front and back face sides have to be inspected separately with a same head unit, thereby the disk once loaded has to be unloaded and loaded after turning it over which reduces the inspection efficiency.